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Can I measure signal intensity received by MC3361BP

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DamoRC

Member
I have an electric RC car. I need to replace the receiver antenna (it was nicked in a crash).
The Rx/Tx operate in FM at 26.995 MHz (crystal oscillator). The stock receiver antenna is 41 cm in length (which is somewhere between 1/32 and 1/16 wavelength). I was wondering if I should replace the antenna with something exactly at the 1/32 or 1/16 wavelengths (34.7 or 69.4 cm respectively).
So I did some searching and discovered that the antennae are not necessarily exact fractions of wavelength depending on the circuitry used by the receiver (the explanations of which were beyond my grasp).
Then I came across this site where a guy was able to determine some relationships between antennae length, configuration etc by measuring the voltage at an RSSI pin on his receiver chip:
RC-CAM Projects: R/C Antenna Experiments
I thought that it might be interesting (for me at least) to replicate some of the experiments because all he needed was a DVM and some knowledge of the FM demodulator IC (to find the RSSI pin).
So I found the datasheet for the chip that my car uses (MC3361BP) which can be found here:
https://www.utc-ic.com/spec/MC3361BP.pdf
But this chip does not have an RSSI pin.
So my question is whether it is possible to perform the same type of experiments on this chip (using only a DVM) and, if so, how?

As always - thanks in advance

p.s. I know I could just try different antenna lengths and drive the car until it fails to respond - I just thought the approach used in the link would be interesting to replicate.
 

RadioRon

Well-Known Member
Your data sheet link is not for the original maker of the part. this one is better:
https://www.datasheetcatalog.org/datasheet/motorola/MC3361BP.pdf

I don't see any RSSI function in this chip at all, so doing it exactly the same way is out. I'm a bit stuck for an alternative method. One possibility is to use the adjustable squelch control to create a set point and then adjust the antenna to find which range of lengths bring the signal level over that setpoint (which levels "open" the squelch, as we say, as measured as a large change in voltage with a voltmeter at pin 14 of the IC). Once you find the range for a loose setpoint, you adjust the squelch a bit further ("tighter" as we say) and then adjust your antenna again to find which range of lengths still open the squelch. Iterate this a couple of times and you may be able to zero in on the best length. This technique can be tedious, because each time you do a measurement, you have to back away from the equipment so that your body is not degrading or enhancing the antenna's performance, and I mean back away at least 10 feet or so. Plus, to do this you might want to find a scrap telescopic antenna from on old radio so that adjustments in length are convenient.

Then again, if your receiver doesn't have an adjustable squelch potentiometer, I'm not sure what else to suggest.
 

DamoRC

Member
Thanks for the response Ron.

I think that what you have suggested is beyond me as there is no adjustable squelch pot connected to the chip. I'll just have to try the old fashioned way (using car response at increasing distances) and limit the tests to 1/16th, 1/32nd or the stock 41 cm to see which performs best.

Thanks again.
 
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